1. Field of the Invention
The invention relates to a capacitor assembly having a layered capacitor housed in a container, and in particular to means for securing the capacitor in the container.
2. Description of the Prior Art
Capacitor assemblies of the above-described type are typically produced by wrapping alternating conductive and insulative layers around a form, such as a mandrel, and securing the resulting wound capacitor in a protective housing. Ordinarily, the housing is a cylinder which is closed at one end and open at the opposite end. After insertion of a capacitor the open end is sealed by installing a lid.
The capacitor must be secured such that vibration will not cause it to move within the container. Such movement could damage the capacitor by electrically shorting the conductive layers. One means for securing such a layered capacitor within a container is described in U.S. Pat. No. 4,486,809, which is hereby incorporated by reference. In the capacitor assembly described, special cup-shaped members are added to the assembly to anchor the capacitor within the container. Each of these cup-shaped members includes an integral central spike for attaching the member to a respective end of the capacitor. Attachment is achieved by inserting the spikes into opposite ends of a hollow core member around which the capacitor layers are wrapped, thereby centering and holding the capacitor in place.
Such added members are not desirable, because they increase the assembly and material costs of the finished capacitor assembly and occupy valuable space within the container. Such space could otherwise be used to increase the size, and thus the capacitance, of the capacitor. In electrolytic capacitors the space is also needed to allow for the expansion of gases which may be generated during operation.
U.S. Pat. No. 4,245,277, which is hereby incorporated by reference, discloses a capacitor assembly in which a wound capacitor is secured in a cylindrical container between a closed end and a lid installed on an open end. The capacitor is centered and supported within the container by means of a hollow core member, around which the capacitor layers are wrapped, and by first and second centering members projecting into opposite ends of the hollow core member. The first centering member is a cylindrical member extending from the lid, and the second centering member is a truncated cone-shaped member integrally formed in the closed end of the container. The first centering member fits closely within one end of the core member, while the second centering member partially projects into the opposite end of the core member.
This arrangement eliminates the need for the cup-shaped members disclosed in the above-mentioned U.S. Pat. No. 4,486,809, but it still requires the added hollow core member. Also, the truncated cone-shaped member causes wasted space, because contact of the hollow core member with the sloped sides of the cone-shaped member keeps the capacitor spaced from the closed end of the container.
FIG. 1 illustrates another prior art capacitor assembly in which a wound capacitor 10 having a hollow center 11 is secured in a cylindrical container 12, without the use of a central core member. Rather, the capacitor itself is secured between an anchor 14, which is inserted in a closed end of the container, and a molded insulative lid 16, which is attached to an open end of the container. The anchor includes a conical spike 18 and three radially-extending legs 20 for centering the spike in the closed end of the container. The inner side of the lid has a plurality of integrally-molded members including a conical spike 22 and three radially-extending ribs 24. The lid also includes terminals 26 and 28 which are electrically connected to conductive layers of the capacitor by means of respective conductive strips 30 and 32.
In the capacitor's assembled state, the capacitor 10 is compressed between the ribs 24 and the conical spike 18 to secure it in place. The ribs 24 press directly into the edge of the layers at the upper end of the capacitor while the sloped surface of the conical spike 18 presses against the edge of the hollow center 11 at the bottom end of the capacitor. Conical spike 22, projecting from the lid, fits loosely within the upper end of the hollow center and serves only as a means for approximately centering the upper end of the capacitor as the lid is installed onto the container.
The arrangement of parts illustrated in FIG. 1 eliminates the need for the central core member utilized in the above-mentioned patents, but it still requires the added anchor part 14. Consequently, the arrangement suffers from the added assembly and material costs resulting from use of the added anchor part. The arrangement also wastes space in the container because, as in the capacitor assembly disclosed in U.S. Pat. No. 4,245,277, the conical spike 18 tends to keep the capacitor spaced from the closed end of the container.
Even more significant is a problem which has been discovered during vibration testing of the FIG. 1 capacitor assembly. Because the capacitor is relatively soft in comparison to the conical spike 18, during vibration the spike tends to deform the innermost layers of the capacitor and enlarge the area of the hollow center 11 adjacent the spike. This enlargement reduces the compressive holding force applied by the ribs 24, and allows a reciprocatory rubbing motion of the capacitor against the spike and the ribs. This rubbing motion can wear through multiple layers of the capacitor and effect shorting together of conductive layers normally separated by interposed insulative layers. This reduction in compressive holding force is accentuated if the capacitor is of a type which generates significant internal gas pressure during operation. Such gas pressure can cause the closed bottom of the container to deform outwardly, with the maximum deformation occurring at the center of the bottom. This deformation will force the conical spike 18 away from the lid 16 and allow an even greater amplitude of movement of the capacitor during vibration.
U.S. Pat. No. 3,509,427 discloses an early capacitor assembly which avoided many of the above-described disadvantages. The early capacitor assembly comprises a cylindrical container with a closed bottom end, a wound capacitor inserted in the container, a lid closing a top end of the container, and electrical leads passing through the lid and into the capacitor layers. The capacitor is secured in position by longitudinally-extending indentations formed in the side of the container. These indentations are equidistantly spaced around the circumference of the container for centering the capacitor.
This capacitor assembly is simple, but wastes space. The capacitor must be made with a sufficiently large diameter to contact the indentations, even if only a smaller diameter is needed to obtain the capacitance desired. Thus any additional space for internal gas expansion which is obtained by avoiding the use of added capacitor-securing elements is sacrificed by increasing the diameter of the capacitor.